In the application where a metal material has been conventionally used, it is an important technical challenge to achieve weight reduction while maintaining the mechanical property and productivity required in the application by using, instead of a metal material, a fiber-reinforced composite material, particularly a fiber-reinforced composite material including a resin and reinforcing fibers such as a carbon fiber, an aramid fiber or a glass fiber.
For enhancing the mechanical property of the fiber-reinforced composite material, a technique of using a continuous fiber or increasing the fiber volume content ratio (VD is known. In the case of using the continuous fiber in the fiber-reinforced composite material, the mechanical property may be enhanced due to increase of Vf. However, a woven fabric, a unidirectional material, and the like, in which a continuous fiber is used, generally have a problem that fibers are stacked at various angles, for example, at 0°/+45°/−45°/90°, because of the anisotropy of the fibers, and furthermore the stacking process becomes complicated, for example, the fibers are stacked in plane symmetry, so as to prevent warpage of the shaped product, which leads to low productivity. On the other hand, in the case of a fiber-reinforced composite material obtained from a mat-form material of cut fibers, it is difficult to enhance the fiber volume content on account of the presence of fibers in the three-dimensional direction or many fiber entanglements, and the like. In addition, when the mat-form material of cut fibers is used as a reinforcing fiber of a fiber-reinforced composite material, there is a problem that the reinforcing fiber is difficult to sufficiently develop strength enhancement due to discontinuity of the fibers compared with the case of using a continuous fiber, and development rate of strength of the reinforcing fiber in a shaped article is 50% or less relative to the theoretical value. For example, Non-Patent Document 1 describes a fiber-reinforced composite material obtained from a carbon fiber mat by using a thermosetting resin as a matrix, but development rate of strength of such a fiber-reinforced composite material is about 44% relative to the theoretical value.
A fiber-reinforced composite material including a thermosetting resin as a matrix, which has been conventionally proposed, is obtained by subjecting an intermediate base material called a prepreg in which a reinforcing fiber base material is previously impregnated with a thermosetting resin to heating and pressurization by the use of an autoclave, for 2 hours or more depending on the case. In recent years, there has been proposed an RTM method where a reinforcing fiber base material not impregnated with a resin is set in a mold and then a thermosetting resin is infused therein, and the molding time has been greatly shortened. However, even when the RTM method is used, molding of one component requires 10 minutes or more (Patent Document 1).
Accordingly, attention has been focused on a composite material including, as a matrix, a thermoplastic resin in place of the conventional thermosetting resin, particularly, on a composite material including a carbon fiber as a reinforcing fiber (carbon fiber-reinforced composite material). However, the thermoplastic resin requires a long time to impregnate a fiber base material with the resin because of high viscosity compared with a thermosetting resin, as a result, the tact time for molding disadvantageously becomes long. In addition, it is known that a carbon fiber-reinforced composite material having an in-plane isotropy, in which carbon fibers are not oriented in a specific direction in the plane, is preferred, but when forming a composite material by impregnating a carbon fiber mat, or the like, with a thermoplastic resin, a high pressure is required for the impregnation due to high viscosity of the thermoplastic resin in a molten state, and there is a problem that it becomes difficult to maintain the in-plane isotropy by generating disorder of fiber orientation attributed to flow of the fibers and resin as in the molding.